`otlp` exporter uses incorrect path when base endpoint path is non-root

[jfallows]

When base endpoint has a non-root path, such as http://localhost:8080/telemetry, then default overrides for logs and metrics do not append, instead they replace, giving:

• http://localhost:8080/v1/logs instead of http://localhost:8080/telemetry/v1/logs
• http://localhost:8080/v1/metrics instead of http://localhost:8080/telemetry/v1/metrics

Workaround is to explicitly specify overrides to use v1/logs instead of /v1/logs, and v1/metrics instead of /v1/metrics.

Fix should change the default behavior such that default paths are appended to the base endpoint path, even when that path is non-root.

[Kunals990]

@jfallows could you please assign this issue to me ?